Thrust bearing and use of same with apparatus for reducing repetitive stress injury

ABSTRACT

A thrust bearing for bearing a load along a force vector includes at least two roller elements in rolling contact with one another and arranged along the force vector such that the load is borne through the roller elements. The roller elements are retained within a retainer such that they remain aligned along the force vector while subjected to loads. The retainer used to house the roller elements may be designed to abut the roller elements at minimum points of contact, or alternatively to distribute the load borne by one or both of the roller elements across a greater surface area. One application of the preferred thrust bearings is in an apparatus such as a keyboard support for reducing repetitive stress injuries. A main disk is rotatably coupled to a base and supports a support member on a plurality of the thrust bearings. Rotation of the main disk varies the tilt of the support member relative to the base, thereby adjusting the position of a keyboard supported on the support member relative to an operator.

FIELD OF THE INVENTION

The invention is directed to a thrust bearing for bearing a load along aforce vector, and more particularly, is directed to a thrust bearinghaving a pair of roller elements in rolling contact with one anotheralong the force vector. Moreover, the invention is directed to anapparatus for reducing repetitive stress injury incorporating suchthrust bearings to support a movable work surface thereon.

BACKGROUND OF THE INVENTION

Thrust bearings have enumerable applications where a force must be bornealong a force vector between two members, with one member movablerelative to the other. Conventional thrust bearings typically include aplurality of roller elements such as ball bearings housed within racesformed in a pair of opposing members. The opposing members are movablerelative to one another by virtue of the rolling contact of theplurality of roller elements rotating within the races formed betweenthe opposing members.

Due to the relatively large forces which are borne by thrust bearings,reduction in the friction within the bearings is critical to ensurerelative movement of the opposing members while under load. For example,U.S. Pat. No. 4,523,863 to Okoshi discloses a thrust ball bearing usingmultiple ball bearings and having opposing races with slightly largercross-sectional radii to minimize contact, and therefore friction,between the balls and the races.

U.S. Pat. No. 2,244,434 to Shaw discloses a supporting roller bearingwherein a pair of cylindrical rollers are supported by three sets ofanti-friction bearings, with each roller element supporting two of thethree sets of bearings. However, since each roller element engages twoanti-friction bearings, increased friction is exhibited within thesystem. In addition, the offset nature of the rollers relative to theanti-friction bearing applies lateral forces to the rollers, which alsoincreases the friction present in the system.

U.S. Pat. No. 4,297,913 to Garbo discloses a remote control having apush-pull blade with captive rolling elements. The rolling elements arein contact with one another as well as with outer races formed within atubular sheath. The rolling elements are retained within openings in thepush-pull blade such that they are maintained in rolling contact withone another. However, Garbo is not directed to a thrust bearingarrangement. The primary forces exhibited upon the system are along thelongitudinal axis of the tubular sheath, and consequently the rollingelements are arranged along an axis which is orthogonal to the forcevector. In addition, the manner in which the rolling elements areretained by the push-pull blade may not adequately house the rollingelements in operation, and may introduce additional friction in certainapplications.

Therefore, a substantial need exists for a thrust bearing arrangementwhich is capable of permitting the relative movement of two memberswhile bearing a load along a force vector applied between the members.In addition, a substantial need exists for such a bearing arrangementwhich exhibits reduced friction in a simple and economical manner.

SUMMARY OF THE INVENTION

The invention addresses these and other problems associated with theprior art in providing a thrust bearing for bearing a load along a forcevector, in which a pair of roller elements are placed in rolling contactwith one another and are arranged along the force vector. By arrangingthe roller elements in this manner, the load is borne through the rollerelements and lateral forces applied to the roller elements will notsubstantially increase the friction exhibited within the system.

One particular application of the invention is in an apparatus forreducing repetitive stress injury, whereby a plurality of thrustbearings support a support surface in such a manner that movement of thethrust bearings relative to the support surface imparts movement of atleast a portion of the support surface in a direction generally alongthe force vector (e.g., by tilting the support surface). When anoperator performs a repetitive task on or in conjunction with thesupport surface, the apparatus may be used to automatically vary theorientation of the support surface over time relative to the operator,thereby reducing the repetitiveness of the task.

Therefore, according to one aspect of the invention, a thrust bearing isprovided for bearing a load along a force vector. The thrust bearingincludes at least two roller elements in rolling contact with oneanother and arranged along the force vector such that the load is bornethrough the roller elements, and a retainer retaining the rollerelements along the force vector under load.

In accordance with another aspect of the invention, a thrust bearing isprovided for bearing a load along a force vector between first andsecond load bearing members and for permitting relative movement of athird member between the first and second members in a directiongenerally orthogonal to the force vector. The thrust bearing includesfirst and second stacked roller elements retained in the third member.The roller elements are in rolling contact with one another and with thefirst and second members, respectively, and are arranged in the thirdmember along the force vector to bear the load between the first andsecond members.

According to another aspect of the invention, an apparatus is providedfor reducing repetitive stress injuries. The apparatus includes a base;a support surface for supporting a load along a force vector and fromwhich an operator performs a repetitive task; and moving means formoving at least a portion of the support surface relative to the base ina direction generally along the force vector, the moving means includinga rotatable member coupled to the base and having a plurality of thrustbearings supporting the support surface on the base, each thrust bearingincluding top and bottom ball bearings in rolling contact with oneanother and arranged in a direction generally along the force vector,the top ball bearing in rolling contact with the support surface and thebottom ball bearing in rolling contact with the base.

According to an additional aspect of the invention, a keyboard supportis provided, which includes a base having at least two idler gearsrotatably mounted thereon; a drive motor mounted on the base and havinga drive gear rotated thereby; a main gear rotatably coupled to the basebetween the drive gear and the idler gears, the main gear includingfirst, second and third thrust bearings supporting the main gear on thebase, each thrust bearing including first and second ball bearings inrolling contact with one another and arranged generally along a forcevector along which the thrust bearing supports a load, and the firstthrust bearing being taller than and disposed at a different radius fromthe second and third thrust bearings; and a support surface forsupporting a keyboard, the support surface being tiltably mounted to thebase through a flexible member and supported on the first, second andthird thrust bearings, the flexible member resisting rotation of thesupport surface relative to the base, and the support surface includingan annular profile member mounted thereto to support the support surfaceon the first thrust bearing.

These and other advantages and features, which characterize theinvention, are set forth in the claims annexed hereto and forming afurther part hereof. However, for a better understanding of theinvention, and the advantages and objectives attained through its use,reference should be made to the Drawing, and to the accompanyingdescriptive matter, in which there is described preferred embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a side elevational view of a first preferred thrust bearingconsistent with the principles of the invention, shown supporting afirst member on a second member.

FIG. 2 is a top plan view of the first preferred thrust bearing, takingalong line 2--2 of FIG. 1.

FIG. 3 is a side cross-sectional view of the first preferred thrustbearing, taken through line 3--3 of FIG. 2.

FIG. 4 is a cross-sectional view of a second preferred thrust bearingconsistent with the principles of the invention.

FIG. 5 is a partial cross-sectional view of a third preferred thrustbearing consistent with the principles of the invention.

FIG. 6 is a functional side elevational view of one exemplary apparatussuitable for use with the preferred thrust bearings, wherein a topmember is selectively inclined relative to a bottom member.

FIG. 7 is a functional top plan view of another exemplary apparatussuitable for use with the preferred thrust bearings with parts thereofcut away, wherein a top member is selectively elevated relative to abottom member.

FIG. 8 is a functional side elevational view of the apparatus of FIG. 7.

FIG. 9 is a top plan view of a preferred apparatus for reducingrepetitive stress injury which utilizes preferred thrust bearingsconsistent with the invention, with portions of the housing and the topsurface of the support member cut away.

FIG. 10 is a cross-sectional view of the apparatus of FIG. 9, takenthrough line 10--10.

FIG. 11 is a perspective view of one alternative profile member suitablefor use in the apparatus of FIGS. 9 and 10.

FIG. 12 is a perspective view of another alternative profile membersuitable for use in the apparatus of FIGS. 9 and 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to the Drawing, wherein like parts are denoted by like numbersthroughout the several views, FIG. 1 shows a preferred thrust bearing 10which supports a first member 51 on a second member 52 to bear a forcehaving at least a component of which is borne along a force vector 1.The preferred thrust bearing includes a pair of roller elements 12 and14 placed in rolling contact with one another along force vector 1, andretained in such an orientation by a retainer 20. In this configuration,thrust bearing 10 is permitted to move relative to members 51 and 52along direction 3, which is generally orthogonal to vector 1.

By virtue of the rolling contact between roller elements 12 and 14, amovement of thrust bearing 10 in direction 3 causes the relativerotation of roller elements 12 and 14. The top of roller element 14rolls in the same direction as the bottom of roller element 12, therebyreducing the friction between these elements. Moreover, it is believedthat the friction therebetween is also reduced since retention of theroller elements by retainer 20 takes advantage of the "toppling" effectof the top roller element 12 being supported on the bottom rollerelement 14. In general, a rolling element disposed on top of anotherrolling element has a natural tendency to topple, or roll off of theother roller element. It is believed that by retaining the rollerelements, the direction in which the top element will topple iscontrolled, and this tendency is used to the bearing's mechanicaladvantage. It is also believed that the minimal points of contactbetween the roller elements, the retainer and the first and secondmembers (as discussed below) significantly reduces the frictionexhibited by the preferred thrust bearings.

Turning to FIG. 3, roller elements 12 and 14 are preferably aligned withtheir centers of rotation 13, 15 aligned along force vector 1. Theroller elements are preferably spherical balls, although other shapes,such as cylinders, may also be used. However, it has been found thatspherical balls are preferred because the balls are permitted to "float"in any direction relative to one another and minimize any frictiontherebetween.

Roller elements 12 and 14 are preferably constructed of steel or anyother metal, plastic, ceramic, or other material commonly used for ballbearings and the like. In addition, more than two roller elements may beused or "stacked" in this arrangement, although the addition of rollerelements will in general increase the friction and thereby reduce theperformance of the thrust bearing.

Retainer 20 retains roller elements 12 and 14 along the force vectorwhile under load. Retainer 20 preferably includes a top portion 21 and abottom portion 31, for respectively retaining the top and bottom rollerelements 12 and 14. Top and bottom portions 21 and 31 include interiorwalls 24, 34 which define inner chambers for respectively retaining orhousing roller elements 12 and 14. The inner chambers are connected byan interior opening 30 to permit roller elements 12 and 14 to be placedin rolling contact with one another. The inner chambers connectedthrough opening 30 may simply be formed as a single cylindrical chamber.Preferably, however, an inner ridge 28 segregates the inner chambers anddefines interior opening 30.

Retainer 20 preferably houses roller elements 12 and 14, and to thisextent, it may be preferable to include narrowed ridges, e.g. ridges 22and 32, which define external openings having diameters smaller thanthose of the respective roller elements, such that the roller elementsmay be snapped into the retainer, but will not be easily removabletherefrom. Alternatively, the narrowed ridges may be eliminated ineither top or bottom portion 21 or 31 (e.g., as shown in FIGS. 4 and 5)should the surrounding structure in which the thrust bearing is usedsufficiently maintain its respective roller element within retainer 20.In addition, the retainer may be made of multiple parts, e.g., usingremovable caps which may be removed to permit insertion or removal of aroller element from the retainer.

In general, the primary function of retainer 20 is to support rollerelements 12 and 14 in a lateral direction (orthogonal to the forcevector). Therefore, retention of the roller elements along the forcevector need not be provided by retainer 20.

Inner wall 24 of top portion 21 preferably has a cross-sectionalcurvature profile from top to bottom having a radius which is greaterthan that of roller element 12 (which may also include a profile whichis curved opposite to the roller element, i.e., a "negative" radiusrelative to the roller element). In the preferred embodiment, inner wall24 is linear in cross-section (i.e., an infinite radius). By providing alarger radius for the inner wall of this portion, contact between innerwall 24 and roller element 12 is minimized. In the preferred embodimentwhere inner wall 24 is linear in cross-section, this results in contactbetween roller element 12 and inner wall 24 being substantially a singlepoint from top to bottom, defining a perimeter extending along themaximum diameter of roller element 12. In general, it is desirable tominimize any contact between roller element 12 and retainer 20 tominimize friction, which is adequately served by this preferredconstruction.

Alternatively, as shown on thrust bearing 10" in FIG. 5, a separateridge 25 may be provided on inner wall 24" in the upper portion 21" ofretainer 20". This ridge 25 preferably contacts roller element 12" alongits largest perimeter or width, thereby providing a minimum amount ofcontact between the roller element and retainer 20". In certainapplications, however, a linear wall without a separate ridge may bepreferred since any wear of the retainer or the roller element maydisplace the roller element relative to ridge 25, such that the ridge nolonger contacts the point of maximum perimeter of the roller element andthe roller element is permitted to rattle within the retainer.

In either configuration, it may be seen that the top roller element isin contact with only four points through its cross-section. Two pointsof contact abut the inner wall of the retainer and are alignedorthogonal to the force vector through the center point of the rollerelement (i.e., along the maximum perimeter of the roller element). Theother two points of contact abut the other roller element and the memberwhich the thrust bearing supports (e.g., first member 51 in FIG. 1), andare aligned along the force vector through the center point of theroller element. Both lateral and vertical load forces applied to thethrust bearing are consequently borne through the center point of theroller element.

Returning to FIG. 3, inner wall 24 is preferably sized to contact rollerelement 12 substantially around its perimeter (i.e., the diameter ofinner wall 24 is substantially the same as roller element 12). This isprovided to prevent any rattling or lateral displacement of the rollerelement within retainer 20. However, it will be appreciated that somespacing may also be provided in the alternative.

The inner wall 34 of bottom portion 31 preferably has a cross-sectionalcurvature profile from top to bottom with a radius that is substantiallymatched with that of the bottom ball bearing or rolling element 14.Moreover, the inner wall preferably continues on to the lower surface ofridge 28 to form a contiguous surface. The configuration shown in FIG. 3is of unique applicability to a plastic or composite retainer design,since the load borne by the bottom rolling element 14 is distributedacross the upper portion of inner wall 34. This provides a maximumsurface area upon which to bear a force, thereby increasing the loadbearing capacity of the plastic retainer.

In the alternative, especially for retainers constructed of metal orother stronger or more wear resistant materials, the inner wallcircumscribing the lower rolling element may be constructed to minimallycontact the rolling element. For example, as shown in FIG. 4, analternate thrust bearing 10' includes a retainer 20' having a lowerportion 31' with an inner wall 34' which has a cross-sectional curvatureprofile from top to bottom with a radius that is larger than lowerrolling element 14'. This is similar in operation to top portion 21',wherein rolling element 12' contacts inner wall 24' along a maximumperimeter of the rolling element. An optional ridge 28' also segregatesthe two inner chambers formed by walls 24' and 34' in this design.

The outer structure of the retainer will typically vary depending uponthe particular application in which the thrust bearing is used. Forexample, retainer 20 shown in FIGS. 1-3 has an outer cylindrical wallsuch that the retainer may be inserted into a separate sleeve inoperation. However, it will be appreciated that other structure may beprovided on the retainer depending upon the application, or that theretainer may be formed at an integral part of another member.

The preferred thrust bearings generally do not require any lubricationfor most applications. However, any known lubricants may be used in someapplications if desired.

The preferred thrust bearings may be used in a number of applicationswherever relative movement between members under load is desired, e.g.in robotics, manufacturing applications, automation applications,animated displays, etc.

For example, FIG. 1 shows thrust bearing 10 disposed between a firstload bearing member 51 and a second load bearing member 52. The retainer20, which functions as a third member, is permitted to move relative toboth members 51 and 52 in a direction which is generally orthogonal tothe force vector, e.g., in direction 3. During this movement, allcontacting surfaces between roller elements 12 and 14 and members 51 and52 cooperatively move in the same general direction, thereby minimizingany frictional effects between the surfaces.

One particular benefit of this configuration is that movement of thethird member between the first and second members may be used to impartmovement of the first member in a direction generally along the forcevector. Several examples of this application are illustrated in FIGS.6-10.

For example, FIG. 6 shows one apparatus 120 which may be used as a liftfor inclining a first member 121 relative to a second member 122.Members 121 and 122 may be hingedly connected to one another, withmember 121 supported on a thrust bearing 125. Bearing 125 may includeball bearings, or alternatively a pair of rolling cylinders 126 and 127housed within a retainer 128. Movement of retainer 125 in a directiongenerally orthogonal to the force vector tilts member 121 up or down,thereby providing a lifting action. Another member (not shown) may alsobe coupled to member 121 to translate into a purely linear motion alongthe force vector.

FIGS. 7 and 8 illustrates another application suitable for use with thepreferred thrust bearings. An apparatus 130 includes first member 131supported on a third member 133 which is rotatably mounted to a secondmember 132. First member 131 includes a top surface 134 and a bottomsurface formed by surfaces 135a, 135b and 135c. Surfaces 135a-crespectively engage thrust bearings 141, 142, and 143 which are mountedto third member 133. Moreover, surfaces 135a-c are identicallyconfigured and evenly spaced about the perimeter of first member 131.Surfaces 135a-c have matched degrees of inclination such that, uponrotation of third member 133, top surface 134 of member 131 will raiseand lower in a purely linear motion. It will be appreciated that threepoints define a plane, and therefore as few as three thrust bearings maybe used to support member 131, although more may be used if desired.Correspondingly, a greater number of bottom surfaces such as surfaces135a-c would be required to match the number of thrust bearings used.Furthermore, the degrees of inclination of surfaces 135a-c may be variedto control both the rate and the amount of displacement of first member131 along the force vector. Furthermore, the surfaces may have differentcontours to provide more complex motion. Also, member 133 may onlyrotate through a narrow arc in one or both directions such that eachthrust bearing will circumscribe only one of surfaces 135a-c.

FIGS. 9 and 10 illustrate another apparatus 150 which may be used toreduce repetitive stress injuries and the like, where in generalrotation of a third or rotatable member 182 in a moving mechanism 180between a first, or support, member 160 and a second member or base 170moves at least a portion of the support member relative to the base in adirection generally along the force vector (e.g., by imparting a tiltingmovement to the support member). For the purposes of the invention, adirection "generally along the force vector" should be understood toinclude directions coincidental with the force vector, as well asdirections parallel to the force vector.

One particular application of this structure is in a keyboard support orstand for reducing repetitive stress injuries associated with typing ona keyboard supported thereby. Various keyboard support designs providingthis function are disclosed in U.S. Ser. No. 08/376,584, filed Jan. 23,1995 by Robert J. Crosson, which is incorporated by reference herein.However, apparatus 150 generally includes three thrust bearingsconsistent with the invention in substitution for the pair of wedges andtracks of ball bearings disclosed in the aforementioned application,thereby substantially reducing the number of parts and the complexity ofthe device, while retaining minimal friction during rotations even underheavy load.

Support member, or surface, 160 of apparatus 150 preferably supports akeyboard (e.g., keyboard 250 shown in phantom in FIG. 9) and includes atop surface 162 which is preferably generally square in shape with frontand back edges cut away as at 163 to permit the member to pivot forwardor backward without contacting any surrounding structure in theapparatus. In the preferred embodiment, support member 160 generallytakes the form of an "I" beam, to provide additional support for akeyboard from front to back. In addition, to secure a keyboard to thesupport member, suitable fastening means, such as patches of hook andloop fastener 169, may be disposed on top surface 162 (with matchinghook and loop fastener material being similarly secured to the undersideof a keyboard). Similar cut outs to those on the front and back edgesmay be provided on the side edges if desired, and in general, thecontour of the support member may vary depending upon aesthetic andother concerns.

Support member 160 includes a backing ring 164 which is secured to theunderside of the top surface with fasteners (e.g., screws 166) or inother manners known in the art (e.g., using adhesives, or by beingintegrally molded therein). In addition, a flexible boot or member 165(preferably formed of rubber) is secured to backing ring 164. Supportmember 160 is secured to base 170 by securing a fastener 185 to athreaded axle 184 projecting through flexible boot 165 such that themember is tiltably mounted (i.e., is allowed to tilt relative to thebase) but is not permitted to rotate about the axle.

Support member 160 and base 170 are preferably constructed of sheetmetal or another suitable material such as a plastic or other metals. Itwill be appreciated that the specific design of these members may varydepending upon aesthetic and/or functional considerations.

Moving mechanism 180 includes a main gear or disk 182 which functions asa rotatable member having a plurality of teeth disposed about itsperimeter. Disk 182 houses three supporting thrust bearings 191, 194 and197, and is retained between a drive gear 176 on drive motor 175 and apair of idler gears 186 and 188. Drive motor 175 is preferably hidden bya housing 172 on base 170, and is preferably a low speed motor such as atiming motor, which, in conjunction with the relative sizes of drive andmain gears 176 and 182, provides an extremely slow rotation of disk 182(preferably generating a nearly or fully imperceptible movement ofsupport member 160). The preferred motor is a Model 4 timing motoravailable from Mallory having a speed of 1 RPM and torque of 120Oz-inches. Motor 175 also includes gear reduction, preferably 10:1, torotate disk 182 at 0.1 RPM. By virtue of the reduced friction providedby the preferred thrust bearings, a low torque motor such as a timingmotor is still capable of rotating disk 182 even when a great load isapplied to support member 160. It will be appreciated, however, thatother motors and gear reductions may also be used, and further thatdifferent manners of supporting and driving disk 182, including the useof belts, chains, other gear arrangements, etc., may also be used.

Bearings 191, 194 and 197 each include retainers 192, 195 and 198 whichare preferably formed integrally in disk 182 (e.g., by molding disk 182from plastic), and including suitable internal chambers (e.g., as shownin FIGS. 1-3). The ball bearings in each thrust bearing are aligned orstacked in along a direction which is generally along the force vectorapplied by a load supported on the support member(e.g., the direction ofgravity when the base is placed on a level surface).

It may be preferable to include external surfaces on the retainers whichare somewhat conical in shape to avoid hitting any surrounding structureon support member 160 when it is tilted, as shown in FIGS. 9 and 10. Inaddition, it may be desirable to include additional supporting structure(e.g., braces 190 on retainer 192) to increase the rigidity of theapparatus.

Bearing 191 is preferably relatively taller than (i.e. has a greaterlength along the force vector) and disposed along a different radius r₁from bearings 194 and 197 (at radius r₂). By making this bearing taller(e.g., by using larger ball bearings), the plane defined by the top ballbearings 193, 196 and 199 is inclined relative to horizontal such thatrotation of disk 182 varies the tilt angle of this defined plane, andtherefore support member 160. In addition, by placing this bearing at adifferent radius than the other bearings (such that it travels along adifferent arc), bearing 191 may support a separate annular profilemember 168 secured to the underside of support member 160, whilebearings 194 and 197 support member 160 directly. This providesadditional tilt to member 160, and also permits a customized tiltprofile to be developed for support member 160 to follow.

Profile member 168 is preferably removably secured to support member 160(e.g., using screws or bolts, snap in attachment, etc.), although member168 may also be permanently secured to the support member. However, bymaking the member removable, different profile members may be installedfor different applications.

For example, profile member 168 in FIG. 10 has a constant contour andprovides a generally constant tilt profile. Two alternative profilemembers are shown in FIGS. 11 and 12. Profile member 200 in FIG. 11 iswedge shaped, being generally thicker at the front point 202 than theback point 204 (where front point 202 is installed proximate the frontof apparatus 150--distal from the drive motor). This design may beuseful in conjunction with keyboards that are themselves tilted, sincethe wedge shape tends to minimize the amount the support surface istilted forward (i.e., down in the front) relative to the backwardtilting of the support member.

Profile member 210 in FIG. 12 has a "reverse wedge" shape where thefront and back points 212 and 214 are thicker than the side points 216.This design has the effect of reducing the degree of tilt from side toside relative to the front to back tilt of the support member. It willbe appreciated that a keyboard is generally longer from side to sidethan from front to back, and thus a similar degree of tilt from side toside will displace the outer points of a keyboard a greater distancethan will a similar degree of tilt from front to back. Accordingly, byreducing the degree of tilt from side to side, the actual displacementof the keyboard may be more consistent in all directions. Other profilemember designs may be used in the alternative to provide different tiltprofiles.

Various modifications to the preferred apparatus may be made consistentwith the invention. For example, bearings 191, 194 and 197 may all besimilarly or differently sized to one another, and may be disposed atsimilar or different radii. Moreover, more than three bearings may beused, and any bearing may rely on a common or a separate profile member(or none) to generate different tilt profiles. Other modifications willbe apparent to one skilled in the art.

The above examples are illustrative of several of the many uses of thepreferred thrust bearings in providing relative movement between twomembers subjected to load. However, the above examples are merelyillustrative, and one skilled in the art will appreciate that manydifferent applications may be foreseen for the preferred bearings.Moreover, one skilled in the art will appreciate that variousmodifications and changes may be made to the preferred embodimentswithout departing from the spirit and scope of the invention. Therefore,the invention lies in the claims hereinafter appended.

What is claimed is:
 1. An apparatus for reducing repetitive stressinjuries, comprising:(a) a base; (b) a support member for supporting aload along a force vector and from which an operator performs arepetitive task; and (c) moving means for moving at least a portion ofthe support member relative to the base in a direction generally alongthe force vector, the moving means including a rotatable member coupledto the base and having a plurality of thrust bearings supporting thesupport member on the base, each thrust bearing including top and bottomball bearings in rolling contact with one another and arranged in adirection generally along the force vector, the top ball bearing inrolling contact with the support member and the bottom ball bearing inrolling contact with the base.
 2. The apparatus of claim 1, wherein theplurality of thrust bearings includes first, second and third thrustbearings, and wherein the first thrust bearing is taller than the secondand third thrust bearings.
 3. The apparatus of claim 1, wherein theplurality of thrust bearings includes first, second and third thrustbearings, wherein the first thrust bearing is disposed to move along afirst arc defined at a first radius on the rotatable member, wherein thesecond and third thrust bearings are disposed to move along a second arcdefined at a second radius on the rotatable member, and wherein thesupport member includes a profile member opposing the first arc tocontact the first thrust bearing and provide a desired tilt profile forthe support member.
 4. A keyboard support, comprising:(a) a base havingat least two idler gears rotatably mounted thereon; (b) a drive motormounted on the base and having a drive gear rotated thereby; (c) a maingear rotatably coupled to the base between the drive gear and the idlergears, the main gear including first, second and third thrust bearingssupporting the main gear on the base, each thrust bearing includingfirst and second ball bearings in rolling contact with one another andarranged generally along a force vector along which the thrust bearingsupports a load, and the first thrust bearing being taller than anddisposed at a different radius from the second and third thrustbearings; and (d) a support member for supporting a keyboard, thesupport member being tiltably mounted to the base through a flexiblemember and supported on the first, second and third thrust bearings, theflexible member resisting rotation of the support member relative to thebase, and the support member including an annular profile member mountedthereto to support the support member on the first thrust bearing. 5.The keyboard support of claim 4, wherein the profile member is wedgeshaped, having a greater thickness proximate a front of the keyboardsupport; whereby the degree of forward tilting of the support member isreduced relative to the degree of backward tilting.
 6. The keyboardsupport of claim 4, wherein the profile member has a greater thicknessproximate a front and a back of the keyboard support than proximatesides of the keyboard support; whereby the degree of side to sidetilting of the support member is reduced relative to the degree of frontto back tilting.
 7. The apparatus of claim 1, wherein each thrustbearing further includes a retainer including top and bottom portionsfor respectively retaining the top and bottom ball bearings in thethrust bearing, the top and bottom portions of each retainer defininginner chambers connected by an interior opening such that the ballbearings contact one another through the interior opening.
 8. Theapparatus of claim 7, wherein the top and bottom portions of eachretainer each include an external opening defined by a narrowed ridge,the external opening having a diameter smaller than that of itsrespective ball bearing to restrict removal of the ball bearing from theretainer.
 9. The apparatus of claim 7, wherein the top portion of eachretainer includes an inner wall having a curvature from top to bottomwith a radius which is greater than that of the top ball bearing;whereby contact between the inner wall and the top ball bearing isminimized.
 10. The apparatus of claim 9, wherein the inner wall of thetop portion of each retainer includes a ridge projecting inward tocontact the top ball bearing about a perimeter defined along the maximumdiameter of the top ball bearing.
 11. The apparatus of claim 7, whereinthe bottom portion of each retainer includes an inner wall having acurvature from top to bottom with a radius which is greater than that ofthe bottom ball bearing; whereby contact between the inner wall and thebottom ball bearing is minimized.
 12. The apparatus of claim 7, whereineach retainer includes an inner ridge defining the interior opening andsegregating the inner chambers of the top and bottom portions, andwherein the bottom portion of each retainer includes an inner wall withan upper portion with a curvature from top to bottom having a radiuswhich is substantially the same as that of the bottom ball bearing;whereby the load borne by the bottom ball bearing is distributed acrossthe upper portion of the bottom portion inner wall.
 13. The apparatus ofclaim 12, wherein the inner ridge between the top and bottom portions ofeach retainer is contiguous with the upper portion of the bottom portioninner wall to bear a portion of the load borne by the bottom ballbearing.
 14. The apparatus of claim 7, wherein each retainer is plastic.15. The apparatus of claim 7, wherein each retainer is metal.
 16. Theapparatus of claim 7, wherein the top ball bearing of each thrustbearing contacts the retainer solely about a perimeter defined along themaximum diameter of the top ball bearing.
 17. The keyboard support ofclaim 4, wherein the first ball bearing in each thrust bearing is a topball bearing in rolling contact with the annular profile member, whereinthe second ball bearing in each thrust bearing is a bottom ball bearingin rolling contact with the base, and wherein each thrust bearingfurther includes a retainer including top and bottom portions forrespectively retaining the top and bottom ball bearings in the thrustbearing, the top and bottom portions of each retainer defining innerchambers connected by an interior opening such that the ball bearingscontact one another through the interior opening.
 18. The keyboardsupport of claim 17, wherein the top and bottom portions of eachretainer each include an external opening defined by a narrowed ridge,the external opening having a diameter smaller than that of itsrespective ball bearing to restrict removal of the ball bearing from theretainer.
 19. The keyboard support of claim 17, wherein the top portionof each retainer includes an inner wall having a curvature from top tobottom with a radius which is greater than that of the top ball bearing;whereby contact between the inner wall and the top ball bearing isminimized.
 20. The keyboard support of claim 19, wherein the inner wallof the top portion of each retainer includes a ridge projecting inwardto contact the top ball bearing about a perimeter defined along themaximum diameter of the top ball bearing.
 21. The keyboard support ofclaim 17, wherein the bottom portion of each retainer includes an innerwall having a curvature from top to bottom with a radius which isgreater than that of the bottom ball bearing; whereby contact betweenthe inner wall and the bottom ball bearing is minimized.
 22. Thekeyboard support of claim 17, wherein each retainer includes an innerridge defining the interior opening and segregating the inner chambersof the top and bottom portions, and wherein the bottom portion of eachretainer includes an inner wall with an upper portion with a curvaturefrom top to bottom having a radius which is substantially the same asthat of the bottom ball bearing; whereby the load borne by the bottomball bearing is distributed across the upper portion of the bottomportion inner wall.
 23. The keyboard support of claim 22, wherein theinner ridge between the top and bottom portions of each retainer iscontiguous with the upper portion of the bottom portion inner wall tobear a portion of the load borne by the bottom ball bearing.
 24. Thekeyboard support of claim 17, wherein each retainer is plastic.
 25. Thekeyboard support of claim 17, wherein each retainer is metal.
 26. Thekeyboard support of claim 17, wherein the top ball bearing of eachthrust bearing contacts the retainer solely about a perimeter definedalong the maximum diameter of the top ball bearing.